I don't get it also, try him somewhere else ffs. We have tried Walker in the midfield, why not give Jack a chance there? Is the idea so disgusting or abominable or what? It wouldn't take anything away from the manager if he tries Jack there and it works out.
I think there's some concern that if he's played there and he shines, then the pundits might say "See, we've always said it that the midfield was his natural position all along and he's manager has been wasting him on the wing all these years", I don't think that's a situation Pep wants to find himself in.
I'm m just going to say it, at this point it feels like an ego thing rather than a decision based on footballing reasons. Pep doesn't like being proven wrong and for things to remain that way, Jack would have to keep playing on the wing.